This invention relates to a fluidized bed reactor and, more particularly, to such a reactor in which heat is generated by the combustion of particulate fuel in a fluidized bed.
Fluidized bed reactors, usually in the form of combustors, boilers, gasifiers, or steam generators, are well known. In a normal fluidized bed arrangement, air is passed through a perforated plate, or grate, which supports a bed of particulate material, usually including a mixture of fuel material, such as high sulfur bituminous coal, and an adsorbent material for the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. In addition to enjoying a high capability for reducing the amount of sulfur in the gases introduced to the atmosphere, such an arrangement permits relatively high heat transfer rates per unit size, substantially uniform bed temperatures, relatively low combustion temperatures and reduction in corrosion and boiler fouling.
In the fluidized bed combustion process, the coal and adsorbent are continuously introduced into the bed by suitable feeders, injectors, or the like and the spent coal and adsorbent are discharged from the lower portion of the bed, usually through a gravity drain pipe extending through a wall of the heat exchanger or through a discharge opening.
In order to optimize operating conditions, the fluidized bed has often been divided into a plurality of zones, usually by selectively introducing the fluidizing air into certain portions of the bed at different times. This enables selected zones to be fluidized while others are dormant to accomodate changing load or start-up conditions.
For example, this selective, or zonal, fluidization simplifies start-up since only a discrete zone of the bed need be preheated to support ignition, which will then readily propogate to adjacent zones as they are brought into service. Also, load control may also be easily achieved by zonal fluidization by increasing or decreasing the number of zones which are fluidized. Further, zonal fluidization enables hot gases to back flow from operating zones through the dormant beds for bed temperature maintenance prior to rapid light-off and ignition of adjacent bed sections.
However, when zonal fluidization is utilized, the area of the bed which is fluidized tends to expand in height above the grid. As a result, the flow of air and the gaseous products of combustion through the fluidized bed causes the particulate materials to tend to build up, or mound, on the unfluidized portion of the bed. This mounting, of course, destroys the parameters under which the bed operates, and can severely effect its efficiency.